This invention relates to digital circuits and, more particularly, to high-speed optical digital circuits.
Prior art digital circuits mostly rely on electronic technology. Currents are induced in various materials through the application of electric fields, and the flowing currents that are induced combine with passive elements and nonlinear active elements to result in logic functions. Examples of such circuits are CMOS logic gates made by combinations of silicon FETs. Other semiconductors, such as gallium arsenide can be used as well. In an effort to increase the operational speed of such digital circuits, work has been carried out recently on electro-optic devices. The SEED device (Self Electro-optic Effect Device) is one such device. It is a surface device to which a light beam is applied and from which the applied light reflects or fails to reflect. The control mechanism that determines whether the SEED reflects or does not reflect comes by means of an electric field which is applied to the device and which generates carriers that affect the reflectivity of the SEED device. Thus the optical response of the SEED is modifiable through electronic control and although the optical path through the device may support a very large information bandwidth, the overall speed of operation may be limited by the speed of the control signal.
To gain access to a greater bandwidth that is inherent in optical signals, it is necessary to develop logic devices that are all-optical. One class of all-optical logic devices that have been developed rely on interferometric techniques that employ two distinct signal paths. One such example is a Mach-Zehnder interferometer which accepts a signal, splits it into two parts that are sent over two distinct paths, and recombines the signals of the two paths. The "switching" action is achieved by including a phase shifting element in one of the two paths. This element induces a phase shift that is controlled by the application of light and results in a constructive or destructive interference at the point where the two signals are recombined.
One problem with Mach-Zehnder devices with light-controlled phase-shifting elements is that light of a substantial intensity may be required over a long length of the phase shifting element. Consequently, the two paths in such Mach-Zehnder devices do not occupy the same space and are, therefore, subject to different temperatures, pressures, electric fields and other extraneous factors. As a result, the constructive and destructive combining at the output of such devices cannot be reliably controlled.
In an attempt to overcome the problems associated with using two distinct paths in Mach-Zehnder interferometer devices, M. J. LaGasse et al. describe a single fiber interferometer in an article titled "Ultrafast Switching with a Single-Fiber Interferometer", Optics Letters, Mar. 15, 1989, Vol. 14, No. 6, pp. 311-313. They describe an arrangement whereby a single circularly polarized pulse is split into two orthogonally polarized pulses, one of the pulses is slightly delayed, and the two resulting pulses are additively combined to form a pulse pair. The pulse pair is sent along a fiber which has the characteristic that the propagation speed through the fiber changes with the intensity of the beam that passes through the fiber. By adding a strong "pump" pulse that is timed to coincide with, say, the first pulse in the pulse pair, the phase shift of only that pulse is affected. Absent the "pump" pulse, the pulse pairs interfere constructively to form a single, more intense, linearly-polarized pulse. In the presence of the "pump" pulse, however, the combination results in destructive interference and no output pulse is produced. The fact that the two pulses travel down the fiber together helps to compensate against temperature changes, as the propagation delay for both interfering pulses is ideally the same.
One of the problems with this device is that the two pulses are of orthogonal linear polarizations, and this relationship must be maintained so that they can be effectively split and recombined at the output. One would like to use polarization-holding fiber for this purpose, but this device would not give the desired immunity to ambient temperature fluctuations since nearly all polarization-maintaining fiber has a birefringence which is highly temperature dependent. This device has another problem in the extraction of the "pump" signal which has the same polarization as the device output. This makes the device difficult to cascade. A third problem is the fact that the signal is split, one arm is delayed and has its polarization rotated, and the two signals are recombined at the output. This is in effect a Mach Zehnder, so it willonce again be susceptible to ambient conditions.
Another, single fiber, device relies on the interaction between pulses known as solitons. Solitons have the characteristic that they do not broaden when traveling through lengths of fiber. A logic device using solitons proposed by Islam et al. in an article titled "All-Optical Cascadable NOR Gate with Gain," Optics Letters, Vol. 15, pp. 417, 1990, uses the interaction of solitons traveling on the fast and slow axes of a birefringent fiber. Since the speeds of the two solitons (traveling on the different axes) are unequal, solitons which are injected with a slight time delay may actually catch up to each other, and when they do they capture, or trap, each other. The resulting trapped soliton pair propagates with a velocity slower than the original fast soliton. Thus, by observing the output at just the right time, one can detect the presence of the slow soliton by detecting the presence of some dragging of the fast soliton. Thus, the slow soliton acts as a control signal. The problem with this approach is that the dragging is only very slight, and other effects such as temperature fluctuations of the fiber are actually comparable in size to the soliton dragging time. Also, the output pulse must be sampled at precisely the right time, and the variability due to environment uncertainties makes the measurement unreliable even if the time when measurement is to be made is known with the requisite precision.
In a completely separate field of art, one that deals with gyroscopes, a device known as a Sagnac interferometer is used to measure rotation. The Sagnac interferometer comprises a fiber loop that has both ends of the fiber connected to the two output ports of a four port fiber directional coupler. Light is injected through one of the coupler's unused ports into the fiber. The coupler causes the injected light to be split, with one portion of the light traveling through the fiber loop in a clockwise direction, and the remaining portion of the light traveling through the fiber loop in a counter-clockwise direction. The light portions return to the coupler and recombine therein. Under normal circumstances, (when the gyro is not moving), the light is recombined in the coupler and completely reflected back to the source. The other unused port of coupler receives no light at all. When the fiber loop is rotated, the rotational movement causes the light that travels in one direction to re-enter the coupler slightly ahead of the light that travels in the opposite direction. The resultant combining of the light is different than before and, consequently, the light is no longer fully reflected to its source. This permits a measurement of the rotational movement of the gyroscope.
Employing the principles of the Sagnac interferometer, in an article titled "Pulsed-Mode Laser Sagnac Interferometry with Applications in Nonlinear Optics and Optical Switching", Applied Optics, Vol. 25, No. 2, January 1986, pp. 209-214, Li et al. describe an experimental setup employing a beam splitter and three mirrors that form an optical, square loop. In one of the loop legs adjacent to the beam splitter they include an element that is capable of induced refractive-index changes. A light pulse is applied to the beam splitter which causes one pulse to travel in a clockwise direction through the loop and another pulse to travel in a counter-clockwise direction through the loop. Still another pulse is derived from the original pulse to serve as a "pump" pulse that is separately applied to the variable refractive-index element. The two pulses that travel around the loop and meet at the beam splitter either combine constructively, when the "pump" pulse is absent, or destructively, when the "pump" pulse is present and is timed to coincide with presence of one of the pulses within the variable refractive-index element.
This arrangement has practical drawbacks in that it requires a beam splitter, three mirrors, a substantial dedication of space, and extreme sensitivity to the timing between the "pump" pulse and the pulses traveling around the loop. No provision is made for the "pump" pulse and the signal pulse to travel together except over the spatial volume in which they overlap in the variable refractive-index medium. Thus a considerable optical intensity will be necessary for the "pump" beam to sufficiently change the refractive index of the medium.
In "Soliton Switching in a Fiber Nonlinear Loop Mirror", Optics Letters, Vol. 14, No. 15, Aug. 1, 1989, pp. 811-813, Islam et al. describe a self-switching Sagnac interferometer. The basis of this arrangement is that the coupler of the Sagnac interferometer splits an incoming signal unevenly. By sending a high intensity signal into the device, the high intensity signal traveling in one direction causes, and concurrently experiences, a large phase shift. However, when a low intensity signal is sent into the device, the phase shift is correspondingly small. The result is that low-intensity signals are essentially reflected to the source, while high-intensity signals are not reflected to the source.
The problem with this arrangement, of course, is that one cannot control the switching, except by external modification of a signal's intensity.
In an article titled "Optical Fiber Switch Employing a Sagnac Interferometer", Applied Physics Letters, Vol. 55, pp. 25, 1989, Farries et al. describe a switch using a Sagnac interferometer with a signal beam of one wavelength and a pump beam of another wavelength. The signal beam is split evenly by the coupler, but the "pump" pulsed beam is injected into the Sagnac loop unevenly. Hence, the signal beam traveling clockwise around the loop experiences a different phase shift than the signal beam traveling counter-clockwise around the loop when the "pump" pulsed beam is present. Consequently, depending upon the presence of the "pump" beam, the signal beams combine destructively or constructively at the coupler.
Since different wavelengths are employed, it is difficult to cascade these devices. Also, the signal beam is not pulsed and, therefore, this device is not readily applicable to digital switching.
In an article entitled "Ultrafast, low power, and highly stable all-optical switching in an all polarization maintaining fiber Sagnac interferometer," Conference record of April 1990 topical Meeting on Photonic Switching, paper 13C-16, M. Jinno and T. Mitsumoto describe a Sagnac arrangement in which a wavelength sensitive polarization holding-coupler was used to couple the pump beam into the loop. This refinement allows a "pump" beam to be coupled into the loop through the input coupler of the interferometer. This device has only two ports, one for the "pump" beam and one for the signal beam. Pulses of different wavelength and different duration were used for the two signals, making the logic device non-cascadable. Polarization shifted fiber was used to ensure that the "pump" and signal pulses travel together.